Our general objective is a complete study of the interaction of metal complexes with nucleic acids--both DNA and RNA. Specifically, we hope to find selective complexes for each of the four purine and pyrimidine nucleosides in the nucleic acid biopolymers by employing both light- and heavy-atom metal complexes. Such labeled nucleic acids would afford an excellent means of sequencing nucleic acids by electron microscope techniques. Our approach to the induction of selectivity is to utilize the various interactions available between nucleosides in the biopolymer and ligands attached to the metal complex. Such a scheme has several advantages: (1) mild reaction conditions can be employed, (2) there is no need for special groups to be introduced into the nucleic acid, (3) a wider range of applicability than other methods of introducing heavy-metal labels into nucleic acids, (4) more than one heavy atom can be incorporated into the complex without modifying specificity. BIBLIOGRAPHIC REFERENCES: Reaction of Coordinated Purines: A Facile, High Yield Route to N(7)-Alkylated Xanthines and Hypoxanthines. The Structure of (Bis(dimethyl-glyoximato)(xanthinato)(tri-n-butylphosphine) cobalt(III)) and the trans-influence in Cobalt(III) Chemistry, L.G. Marzilli, L.A. Epps, T. Sorrell and T.J. Kistenmacher, J. Amer. Chem. Soc., 97, 3351 (1975). Intercalative Stacking Interactions and Interligand Hydrogen Bonding in Metal Purine complexes: the Crystal and Molecular Structure of ((N-salicylidene,N'-methylethylenediamine)(theophyllinato)copper(II)) Monohydrate, t.J. Kistenmacher, D.J. Szalda and L.G. Marzilli, Inorg. Chem., 14, 1686 (1975).